One possible cause is brain differences. Differences in brain structure or neurochemistry may cause body dysmorphic disorder.
Environment is another possible causes. Depending on the environment you live in, life experiences you have, and the culture surrounding you, you may develop body dysmorphic disorder. Some strong potential causes, are the unrealistic expectation of beauty caused by the media, negative social evaluations about your body or self-image, or even childhood neglect or abuse.